Phenol-formaldehyde-urea resin and method of preparation



N Drawing. Filed July 30, 1965, Ser. No. 476,164 7 Claims. (Cl. 260-840) ABSTRACT OF THE DISCLOSURE A phenolic resin is prepared by condensing a mixture containing 2.6 to 4.0 moles (preferably 2.85 to 3.4 moles) of formaldehyde (F) for each combined mole of phenol (P) and urea (U), the urea component amounting to 4 to 17 percent (preferably 9 to 12 percent) by weight (on a charge basis) of the final resin product. When the water miscibility of the reaction product is reduced 3 to -fold, an acidic material is added to reduce the pH to 4 to 7, the resulting acidified reaction mixture is heated to remove excess 'water and to produce a reaction product having a viscosity between S and Z on the Gardner-Holdt scale. The resulting reaction product is useful as a hot-box core binder.

This invention relates to an improved phenolic resin useful as a hot box foundry core binder. In another aspect, it relates to a process for preparing such phenolic resin. In another aspect, it relates to foundry products, such as cores and molds, made from sand or similar aggregate and a novel phenolic resin binder. In another aspect, it relates to a process for preparing such foundry products.

In the foundry art, cores and molds used in making metal castings are generally prepared from shaped, cured mixtures of aggregate material (e.g., sand) and a binder. A host of different materials have been proposed, patented or used as binders for such purpose, including so-called hot box binders, such as various phenolic resins, which require simultaneously heating and molding the sandbinder foundry mix in a core or molding box to convert .the foundry mix into a cured, hard, self-supporting, solid shaped article generally referred to as a core. The instant application is concerned with this general type of hot box binder.

While many of the prior art phenolic resins of the hot box core binder type have enjoyed significant use in the foundry industry, cores and molds made from such binders have a number of shortcomings which are attributable to said binder. For example, in curing the sand-binder foundry mix in the form of a core, noxious formaldehyde fumes often form which necessitates ventilation of the core-making area. Similarly, c-ores made from such binders often have poor hot strength and low tensile strength, and oftentimes castings made with these foundry cores exhibit internal porosity and pitting, due to the evolution of deleterious nitrogen gases generated when the molten metal decomposes the cores.

Accordingly, an object of this invention is to provide an improved phenolic resin useful as a hot box core binder. Another object is to provide an improved process for preparing such a phenolic resin. Another object is to provide improved foundry products such as cores and molds made from sand or similar aggregate and a novel phenolic resin binder. Another object is to provide a process for preparnited States Patent 0 3,404,198 "Patented Oct. 1

ing such improved foundry products. Another object is to provide an improved hot box foundry core binder which imparts to foundry cores or molds made therefrom such desirable properties as high hot strength and high tensile strength. Another object is to provide an improved hot box foundry core binder which will not give rise to noxious formaldehyde fumes when cores and molds made from this binder are cured. Another object is to provide an improved hot box foundry core binder from which improved foundry cores can be made which will not cause pitting or other imperfections in metal castings made therefrom. Further objects and advantages of this invention will become apparent to those skilled in the art from the following descriptions and appended claims.

Briefly, according to this invention, improved hot box foundry core binders are made by condensing in a single step under alkaline conditions a mixture containing phenol, formaldehyde, and urea, in certain critical relative proportions, at elevated temperatures until the liquid resin reaction mixture has a desirable low degree of water miscibility, thereafter acidifying the liquid reaction mixture and removing sufficient water therefrom until the liquid reaction mixture has a desirably high viscosity, and cooling and recovering the resulting partially water soluble, liquid phenolic resin product.

According to this invention, it is essential that the mixture which is condensed have certain critical relative proportions of resin-forming components in order that the resulting resin product have the properties which make it function satisfactorily as a hot box foundry core binder. Generally speaking, the mixture which is condensed contains 2.6 to 4.0 m-oles (preferably 2.85 to 3.4 moles) of formaldehyde (F) for each combined mole of phenol (P) and urea (U), the urea component amounting to 417% (preferably 9l2%) by weight (on a charge basis) of the final resin product. Stated otherwise, the mole ratio F/ (P+ U) is generally in the range of 2.6 to 4.0 (preferably 2.85 to 3.4), the urea component being 4-l7% (Preferably 9-12%) by weight (on a charge basis) of the resin product.

Although the improved hot box foundry core binder of this invention can be made by using aqueous formaldehyde, e.g., 37% Formalin, together with phenol and urea, the use of a phenol-formaldehyde blend is preferred because the use of aqueous formaldehyde requires removal from the reaction mixture of a large amount of water. The use of a blend of phenol and formaldehyde, rather than aqueous formaldehyde and phenol, avoids the necessity of removing a large amount of water during the resin formation. In addition, the phenol in the phenolformaldehyde blend acts to stabilize the formaldehyde and permit its storage without the difficulties encountered when using aqueous formaldehyde (which usually necessitates incorporating methanol in the aqueous formaldehyde for purposes of stabilization). The phenolformaldehyde blend is stable for long periods of time; it has been observed that no change in the composition of the blend takes place even after three months storage at room temperature or one monthstorage at F. The amount of phenol in the phenolformaldehyde blend is from 20 to 50 weight precent, preferably 28-35 weight percent of the blend, and the balance is formaldehyde and water; if the phenol content is lower than 20 weight percent, it does not impart stability to the formaldehyde. In addition to the phenol-formaldehyde blend, an aqueous blend of urea-formaldehyde is preferred in carrying-out the condensation reaction of this invention. In such aqueous urea-formaldehyde blends, the molar ratios of total available urea to available formaldehyde will generally be in the range of /2 to /s, the urea-formalde blend containing 10 to 25 weight percent water. Such a blend (or concentrate) contains formaldehyde, urea, and equilibrium reaction products of these materials, though the blend itself is a non-polymerized aqueous mixture. A typical commercially available urea-formaldehyde blend contains 60 weight percent formaldehyde, 25 weight percent urea and 15 weight percent water. In using the phenol-formaldehyde blerid and the urea-formaldehyde blend as resinforming' components, the relative amounts of these materials, as well as the concentrations of their components, are such as to meet the aforementioned critical relative proportions of resin-forming components.

In carrying out the condensation reaction of this invention, part of the phenol resin-forming component can be substituted by other phenolic compounds such as cresol, xylenol, cresylic acid, alkyl phenols such as tertiary butyl phenols, amyl phenols, and the like, including mixtures thereof. Also, a part of the urea resinforming component can be substituted by thiourea,, melamine, and the like, and part of the formaldehyde can be substituted by other aldehydes, such as acetaldehyde, or compounds yielding aldehydes, such as para-formaldehyde, and the like.

In order to maintain alkaline conditions during the condensation reaction, any inorganic alkaline or basic material soluble in water can be used, such as sodium hydroxide, sodium carbonate, sodium sulfite, potassium hydroxide, potassium carbonate, barium hydroxide, ammonium hydroxide, and the like. The amount of alkaline material used will be that suflicient to render the condensation reaction mixture alkaline, preferably in the pH range of 8-13. Generally, this alkaline reaction condition can be obtained by using 220, preferably 25%, by weight of the phenol component, Among the bases, sodium hydroxide will generally be found to be the most efiicient alkaline material for this purpose.

The condensation reaction is carried out at elevated temperatures conducive to the formation of the desirable phenolic product; generally, the reaction temperature will be between 80 and 100 (3., preferably between 95 and 100 C., which temperature is preferably reached as rapidly as practical and is maintained long enough to obtain the desired water miscibility. The reaction mixture is preferably heated to reflux, using just enough heat to maintain a gentle refluxing action. This heating or condensation reaction is continued until the water miscibility of the reaction mixture decreases to a desired level. This desired water miscibility, or water tolerance, preferably will be that point at which the water miscibility of a sample of the reaction mixture has been reduced 3 to 5-fold from that extant at the beginning of the reaction (when the initial reaction mixture is completely water soluble). This water miscibility can be determined by taking a sample of the resin reaction mixture and diluting it with successively added volumes of water until the resinwater mixture turns cloudy or displays milkiness; for example, if cloudiness occurs after six parts of water have been added to one part of the resin reaction mixture, the water miscibility of the latter is said to be 6. Alternatively expressed, the condensation reaction is preferably continued until the resin reaction mixture has a water tolerance between about 300 and 500%; for example, when a sample of the resin reaction mixture becomes cloudy when diluted with five times its volume of water,

7 it has a water tolerance of 500%. If the condensation reaction is terminated before the desired degree of water miscibility is obtained, or after the desired water miscibility is obtained, the resin reaction product will not have the optmum properties which make it most useful as a hot box foundry core binder.

Upon carrying out the condensation reaction to the desired water miscibility level, the reaction mixture is then neutralized or acidified with an acidic material such as sulfuric acid, phosphoric acid, hydrochloric acid, phenol sulfonic acid, para-toluenesulfonic acid, maleic acid, oxalic acid, citric acid, tartaric acid, phthalic acid, or the like. Sufiicient acid is added to reduce the pH of the system to 4.0-7.0, preferably 5.5 to 6.5. Neutralization or acidification of the alkaline reaction mixture can be performed while the reaction mixture is still at or near reaction temperature or can be performed after the mixture has been rapidly cooled to 6090 F. If desired, a small amount of glycerol can be added to hasten neutralization.

The neutralized or acidified condensation reaction mixture is then heated at elevated temperatures to remove excess water and increase the viscosity of the resin system. Generally, this second heating step will be carried out at temperatures in the range of -200", preferably 130, under reduced pressure, e.g., 20" to 29 Hg. Heating under these conditions is carried out until the viscosity of the reaction mixture has a value at 25 C. between Q and Z on the well-known Gardner-Holdt scale (e.g., 4.35 to 23 strokes), preferably to a value between U and V on said scale (e.g., 6.2-8.8 stokes). If this second heating or viscosifying step is terminated before or after this desired viscosity increase is obtained, the resin product will not have the optimum properties which make it especially suitable as a hot box foundry core binder. At the completion of this second heating step, the liquid reaction mixture can be cooled and used immediately, if desired, or stored and used later, as a liquid hot box foundry core binder.

The resin product prepared in accordance with this invention is relatively inexpensive to prepare. When used as a hot box foundry core binder, it is relatively thermal stable, comparatively fast curing, and resistant to overcuring. Furthermore, it is resistant to water or Water vapor and will not be affected by conditions of temperature and humidity normally encountered in the foundry. When cured, it -will not liberate excessive amounts of noxious formaldehyde fumes or other gases such as ammonia. Thus, it can be safely used under normal foundry conditions without requiring other than normal ventilation, and castings made from cores bonded with this binder will not exhibit imperfections, e.g., pitting or porosity. The cores made from this binder are non-thermalplastic and will not sag or otherwise deform prior to use. The cured cores also have relatively hard surfaces, that is, they have hot strength and are resistant to abrasion, which properties facilitate the removal of the molded cores from their core box and the handling of the same without breaking, chipping, etc. The cured cores have very good tensile strength and readily collapse from molds after castings have been made. The bench life of the wet sand mix made from the binders of this invention is relatively long and the extent of this 'bench life will be quite sufiicient to enable the foundryman to make up a batch of wet sand mix and mold a sufiicient number of cores with the usual molding presses without hastening such procedure to avoid set-up of the wet sand mix.

The aggregate material used in making the improved foundary products of this invention will typically be sand, though optionally the sand mix can include such materials as iron oxide, ground flakes, fiber wood, cereal, pitch, and the like. The aggregate, e.g., sand, is generally a major constituent, and the binder a minor constituent of the sand mix. Generally, a binding amount of the binder will be used, which amounts to less than 10%, usually between 0.25 and 5%, by weight of the sand. Most often the binder will amount to 1 to 3% by weight of the sand. In addition to the aggregate and binder, the wet sand mix will also have incorporated into it conventional catalysts or hardening agents which cause the binder to crosslink or cure when the shaped sand mix or core is lysts or curing agents which can be used in this invention include acidic catalysts, either diluted or concentrated, such as phosphoric, sulfuric and hydrochloric acids or, more preferably, the salts of strong acids, such as ammonium chloride, aluminum chloride, ammonium bromide, ammonium nitrate, ammonium sulfate, monoor di-amnronium phosphate, and the like. Generally, the amount of catalyst used will be from 0.5 to 3 weight percent of the aggregate, e.g., sand. Other materials wellknown in the art can be also incorporated into the sand mix, such as a small amount of urea, added, for example, for the purpose of absorbing any free formaldehyde that is evolved during curing of the shaped sand mix. Materials whichcan also be incorporated in the sand mix include those used to extend the bench life of the sand mix, such as hexamethylene tetraamine, calcium oxide, zinc oxide, magnesium oxide, aluminum oxide, and the like.

The wet sand mix can be rammed blown or otherwise introduced into pattern and compressed, thereby assuming the shape defined by the adjacent surfaces of the pattern. The molded or shaped sand mix is simultaneously cured in the pattern at elevated temperatures in the range of 200 to 700 F., preferably 350 to 500 F., for varying periods of time, generally from about 5 seconds to about 60 seconds. Temperature and time conditions of the cure will depend upon the particular sand mix being cured, i.e., the particular binder and catalyst and relative proportions thereof in the sand mix. The choice of curing catalysts will in part determine the extent of overcure-degradation of the tensile strength of the cured sand mix at longer curing rates at high temperatures. After the curing of the shaped sand mix or core, the

cured foundry product can be removed from the mold or pattern and thereafter used in a conventional manner in the casting of molten metal, such as cast iron, steel, brass, bronze, aluminum, etc.

The objects and advantages of this invention are further illustrated in the following examples, but it should be understood that the particular materials and the amounts thereof recited in these examples, as well as reaction conditions and other details of these examples, should not be constructed to unduly limit this invention.

Example I An improved resin product of this invention was prepared in the following manner. Into a reaction vessel equipped with a reflux condenser, thermometer and stirrer, 653 grams of a phenol-formaldehyde blend, 225 grams of a urea-formaldehyde concentrate, and 5 grams of 50 wt. percent aqueous sodium hydroxide solution, were added. The phenol-formaldehyde blend used contained 6.25 moles of formaldehyde and 2.50 moles of phenol, in aqueous solution. The urea-formaldehyde concentrate contained 4.5 moles of formaldehyde and 0.94 mole of urea, in aqueous solution. The reaction mixture in the vessel had a pH of 8.4 and was heated from 75 to 212 F. in 40 minutes. After heating it 20 minutes at 212 to 215 F., the water miscibility of the reaction mixture had been reduced from greater than to 4. At this time, heating was discontinued and 22 grams of an acid solutionconsisting of equal parts of 30 wt. percent phosphoric acid and glycerol were added to the reaction vessel, lowering the pH of the reaction mixture to 6.4. The

temperature of the reaction mixture was then cooled to 110 to 120 F. and the excess water distilled under a vacuum of 27" to 28" Hg vacuum. As distillation continued,

' the viscosity 'of the reaction mixture increased, and after it attained a Gardner-Holdt viscosity of U to V (6.3 to 12.1 stokes), distillation was discontinued. During this distillation, 208.5 gra-ms'of distillate was removed, leaving 695 grams of resin product.

- ExampleII In this example, the resin product prepared according to Example I was used as a hot box foundry core binder in one run -(No. 17). Several other runs were also made, using several other resin products of this invention made in a manner similar to that of Example I. A number of other resins of this invention were prepared in a manner similar to that of Example I except that in these other runs, instead of using phenolformaldehyde blend, phenol and para-formaldehyde were used. For purposes of comparison, a number of other resins were prepared, using amounts of resin-forming components outside the scope of this invention. 1

In evaluating said resins products as foundry core binders, in each case sand mixes containing binder were made in a standard Simpson laboratory sand muller. After adding the sand, the curing catalyst and water were added and blended with the sand for 2 minutes, after which the binder was added and blended for 4 minutes. 'In all cases, the finished sand mix comprises 10,000 grams sand (Nugent lake sand), 55 grams catalyst (a mixture of 15 wt. percent ammonium chloride, 79 wt. percent urea, 5 wt. percent hexamethylene tetraamine, and 0.25 wt. percent magnesium oxide), 15 grams water, and 200 grams resin binder. The wet sand mix was taken from the mul-ler and blown with compressed air into an Osborn core maker or heated pattern (hot box) which could be maintained at any temperature between 200 and 600 F. The particular pattern used was a standard AFS 1" tensile briquet (core). At a given pattern temperature (350 or 500 F), the curing of the core was varied between 10 to 30 seconds by opening the core box and withdrawing the briquet from the pattern at the end of the specified time. At the end of'the desired curing time, the briquet was removed from the hot pattern, placed on a table and exposed to ambient laboratory temperautres and allowed to cool at said temperature for at least 1 hour. At the end of this cooling period, the cold tensile strengthof the briquet was determined by using a Dietert standard testing device. In running this tensile strength determination, 6 runs were made and the mean or average value for the 6 runs was recorded.'The hot strength of each of the briquets was also determined, this determination simulating the conditions resulting when a core is lifted from a production hot box resin by ejection pins. If the hot strength of a coreis too low, the ejection pins will break the partially cured shell of the core. The test instrument used for this purpose was a Chatillon pushpull gauge, catalog number 719-40. In carrying out this determination, the recording of time is begun as soon as the air pressure holding the hot box closed is released. After the initial 3 seconds following the opening of the hot box, force is exerted on the gauge; and after another two seconds have elapsed, loading of the gauge begins. Timing is continued until the pin of the gauge penetrates the surface of the briquet. The time of penetration and the maximum load are recorded, the gauge reading in pounds being thereafter converted to p.s.i. The formaldehyde odor level resulting during the curing cycle is determined by removing from the hot pattern a briquet which has been cured 3 seconds at 350 F. and placing it immediately in a tube. A known quantity of carbon dioxide-free airis passed over the briquet and then dubbled through a known quantity of sodium sulfite solution. The formaldehyde present in the air stream is absorbed by the sodium sulfite, liberating sodium hydrovide, which is back-titrated with a standard hydrochloric acid solution to determine theamount of formaldehyde evolved by the hot briquet and absorbed by the sodium sulfite. The formaldehyde odor level is expressed in terms of the number of m1. of 0.100 norm-a1 hydrochloric acid used in titration. In making this determination, the mean value of two determinations was recorded.

:of the casting which were in contact with the cores Table I summarizes the composition of the binder used 2. A method for forming a phenolic resin, which comin these runs and the properties of the curedcores made prises the steps of heating, in the presence of an alkaline therefrom. catalyst, phenol (P), formaldehyde (F), and urea (U),

TABLE I Composition of binder Cured core properties Molar content of charge Tensile strength of cores (p.s.i.) F/(P+U) Urea, \vt.. Formaldehyde Hot Run mole ratio percent of odor lcvel strengt 350 F. cure 500 F. cure I resin Formaldehyde Phenol Urea psi.

11, 7 11,26 5, 50 2, 00' 0 No cure 2 12,1 12,50 5,00 2, 14 0 No cure 10,4 13, 76 5,00 1, 90 0 No cure 10, 1 16, 00 5, 00 2, 08 0 N o cure 11,0 19, 23 5,00 2, 50 60 80 100 0 15, 50 5,00 0 3, 3 157 90 140 190 10, 5 21, 80 5, 00 2, 64 2, 8 313 140 190 250 10, 0 22, 00 5, 00 2, 83 2, 8 304 110 210 350 10, 8 23, 76 5, 00 2, 67 3, 4 373 150 300 440 10, 4 24, 30 5, 00 2, 75 3, 3 313 160 220 340 9, 5 23, 76 5, 00 2, 34 2, 8 295 140 210 310 10, 5 26, 50 5, 00 2, 92 4, 0 625 190 260 310 10, l 27, 00 5, 00 2,83 5, 0 308 210 250 360 9, 4 25, 76 5, 00 2, 50 5, 0 460 140 240 380 9, 6 20, 80 5, 00 2, 50 6, 5 520 190 340 350 4, 17, 76 5, 00 0, 83 3, 5 242 130 240 310 0 21, 50 5,00 1, 87 2,0 266 120 170 260 10, 1 23, 26 5, 00 2, 50 2, 25 330 200 320 390 10, 8 23, 76 5, 00 2, 67 3, 4 373 150 300 440 11,2 23, 76 5,00 2, 58 3, 5 510 250 340 370 12, 6 25, 00 5, 00 3, 10 4, O 720 170 360 320 15, 0 29,00 5, 00 4, 34 3, 7 710 140 310 320 17, 1 33, 00 5, 00 5, 66 4, 0 423 80 200 350 1 In these runs, phenol and para-formaldehyde were used in place of 2 N o cure means sand mix fell apart when hot box was opened.

t he phenol-formaldehyde blend.

Examination of the data of Table I shows that the in amounts sufficient to provide an F/ (P+ U) mole ratio cores made with the binders prepared in accordance of 2.853.4 and produce a resin having 9-12 weight per- With this invention (Runs 7-12 and 16-23) exhibited an cent urea, at a temperature sufiicient to reduce the water acceptable low formaldehyde odor level, i.e., 4 or less, miscibility of the reaction product 3 to 5-fold, adding an had acceptable hot strengths, and desirable high tensile acidic material to reduce the pH of the reaction mixture strengths. Contrariwise, the cores (Runs 1-5) made with to 4.0-7.0, heating the resulting acidified reaction mixture binders having a low F/ (P+ U) mole ratio, i.e., lower at elevated temperatures to remove excess water and prothan 2.85, exhibited low hot strength, did not cure at duce a reaction product having a viscosity between S all at 350 F. and resulted in only low tensile strength and Z on the Gardner-Holdt scale.

when cured at 500 F. In the run where no urea was 3. A resin product comprising the condensation prodused (Run 6), very low tensile strength was obtained. not of phenol (P), formaldehyde (F) and urea (U) Also, the cores (Runs 13, 14 and 15) made with binders having a mole ratio F/(P+U) of 2.85-3.4, containing a g a high F -i- U mole ratio, above 3-4/1 9-12 weight percent urea and having a viscosity on the exhibited prohibitively high formaldehyde odor levels. G rdner-Helm le f Q to Z,

4. A resin product comprising the condensation prod- Example In net of phenol (P), formaldehyde (F) and urea (U) In this example, a cored plate casting was made by having a mole ratio F/(P+U) of containing poring grey iron P cafbon l f l at 9-12 weight percent urea, and having a viscosity on the 2700" F., using cores made according to this invention. Gardncr HO1dt Scale of S to The binder used in making said cores amounted to 3.0 weight percent by weight of the sand, said binder having an F/(P+ U) mole ratio of 3.1/1 and containing 11.0% urea, based on the weight of the resin. The surfaces 5. A resin according to claim 4, wherein the resinforming components are a phenol-formaldehyde blend and an aqueous urea-formaldehyde blend.

6. A method for forming a phenolic resin, which comprises the steps of heating, in the presence of an alkaline catalyst, a blend of phenol and formaldehyde containing from 20 to 50 weight percent phenol, the balance being formaldehyde and water and an aqueous blend of urea and formaldehyde wherein the molar ratio of total available urea to available formaldehyde is in the range of from 1:2 to 1:5, the said urea-formaldehyde blend containing 10 to 25 weight percent water in amounts sufiicient to provide an F /P+U mole ratio of 28523.4 and to produce a resin having 9 to 12 weight percent urea, at a Were good in appearance and exhibited no evidence of pin holes. After the metal was cast, the cores readily collapsed, i.e., they exhibited good shake-out.

Various modifications and alternatives of this invention will become apparent to those skilled in the art from the foregoing description and examples without departing from the scope and spirit of this invention, and it should be understood that this invention is not to be limited unduly to the preferred embodiments set forth herein for illustrative purposes.

I claim: temperature sufficient to reduce the water miscibility of 1. A method for forming a phenolic resin, which com- Ieactlon PmduCt 3 f01d, adding an acidic mateprises the steps of heating in the presence of an alkaline rial to reduce the pH of the reaction mixture at elevated catalyst phenol (1:), formaldehyde (1:), and urea (U), in temperatures to remove excess water and to produce a amounts ffi i t to id an +U l ratio f reaction product having a viscosity between S and Z on 2.85-3.4 and produce a resin having 9-12 weight percent the Gardner-Holdt scale. urea, at a temperature snfiicient to reduce the water 7. A method according to claim 6 wherein the phenol miscibility of the reaction product 3 to 5-fold, adding an formaldehyde blend and the aqueous urea-formaldehyde acidic material to reduce the pH of the reaction mixture blend are reacted at a temperature between and to 0- heating the l'fisulting acidified reaction mixture C. and wherein the acidified reaction mixture is heated at elevated temperatures to remove excess water and proat a temperature within the range f 30 to duce a reaction product having a viscosity between Q and Z on the Gardner-Holdt scale.

(References on following page) References Cited UNITED STATES PATENTS Bender 260-840 Maisch 117-148 Cserny 260-840 Loos 260840 Maisch 117-148 DAlelio 26051.5 Anthony et a1. 260840 Kneipple 161-170 Lang et a1. 260-172 10 FOREIGN PATENTS 998,444 1/1952 Farnce. 557,549 8/1932 Germany.

OTHER REFERENCES 10 WILLIAM H. SHORT, Primary Examiner.

H. SCHAIN, Assistant Examiner. 

